docs: def compute_payload_block_hash(this):
      """Compute hash of each payload block.
      Used to prevent payload corruption and tampering."""
  
      return hmac.new(
          hashlib.sha512(
              struct.pack('<Q', this._index) +
              hashlib.sha512(
                  this._._.header.value.dynamic_header.master_seed.data +
                  this._.transformed_key + b'\x01'
              ).digest()
          ).digest(),
          struct.pack('<Q', this._index) +
          struct.pack('<I', len(this.block_data)) +
          this.block_data, hashlib.sha256
      ).digest()